Printed circuit boards for providing circuit interconnections between various electrical components, such as integrated-circuit packages, resistors, capacitors, and the like are in widespread use. Conventional printed circuit boards generally comprise a flat insulating laminate made of phenolic or a similar material, which has a conductive pattern formed on one or both surfaces. The conductive pattern generally is etched from a layer of copper foil which is adhesively secured to the surface of the laminate board. Production of the pattern is effected by first printing a photoresist image of the desired pattern on the foil and the etching away the exposed portions of the foil not covered by the photoresist. Such boards also are machine drilled or punched to provide openings through the boards to provide mounting holes and openings for passage of conductors from one side of the board to the other (in the case of double sided boards). Once the circuit boards have been completed, the electronic components are attached to the board, and the board then is housed in the particular apparatus with which it is used.
Applications have been developed for molded circuit boards in which a thermoplastic material forming a housing (such as a telephone handset of the like) has a copper foil adhesively secured to the molded housing or body, and a conductive pattern is produced in the manner similar to that employed in the manufacture of conventional printed circuit boards. By adhering the foil for a conductive pattern directly to the housing, the necessity for a separate phenolic printed circuit board enclosed within the housing is eliminated.
An improvement in molded printed circuit boards is disclosed in U.S. Pat. No. 4,584,767 to Vernon C. Gregory. This improvement comprises the formation of a conductive pattern on a flexible plastic film or web which then is placed in a mold prior to formation of a molded housing to cause the web to be securely and integrally bonded to the material used to form the molded housing.
Another approach to providing printed circuit boards in non-planar or multi-planar configurations is disclosed in the Patent to Takahashi U.S. Pat. No. 4,677,252. The circuit board structure of this patent comprises a rigid metal substrate made of aluminum, copper or other suitable material. A first resin layer is bonded to the metal substrate by means of a second resin layer. The two layers are made of different material, with the first layer having a relatively limited elongation. The bonding layer is made of a resin material with an elongation of greater than 100%, and each of the resin layers has a thickness of between twenty (20) to three hundred (300) microns. The total combined thickness of the two layers is less than five hundred (500) microns but at least forty (40) microns a conductive metal layer made of metal foil then is sputtered or plated onto the top of the first layer, with a suitable circuit being etched or otherwise formed in this conductive metal layer.
The circuit board of Takahashi, with the various layers bonded to it, then may be bent into various configurations. The second resinous layer provides stretching between the circuit board and the conductive metal layer without resulting disconnection in the metal layer or the substrate. Because of the substantial thickness of the two resin layers (greater than forty (40) microns), good electrical insulation exists between the conductive metal foil layer and the underlying metal substrate. In addition, however, the resin layers of Takahashi provide considerable thermal insulation as well between the metal substrate and the copper foil circuitry formed on top of the upper resin layer. Thermal transfer through the relatively thick resin layers is poor. The structure of this patent is concerned with bending and is not directed to operation as a thermal heat sink.
Printed circuits made in conjunction with any of the above described techniques are suitable for signal processing semiconductor components rated for operating environments of less than 80.degree. C., for example. For power handling semiconductors, significantly higher operating temperatures are encountered; and it generally is necessary to provide cooling for such semiconductors to prevent their temperatures for rising to the failure point of the devices.
Early semiconductor packages for power semiconductors were designed by electrically connecting the semiconductor die to a metallic housing for the semiconductors. With the die directly attached to the metal of the housing, the package itself served as a thermal conduction path for heat to flow to a lower temperature point, cooling the semiconductor die. In many applications, however, a semiconductor die cannot be mounted in this way on the metallic structure of the package since electrical isolation is required. In such applications, it has been a common practice to mount the semiconductor die on ceramic substrates to achieve the necessary electrical isolation. The most widely used ceramic for circuit substrates is alumina (aluminum oxide), since this ceramic has a very high thermal conductivity for an insulating material. Although other ceramics exhibit greater thermal conductivity, they are not frequently used because of higher costs or handling difficulties. Plastic films (such as used in the Takahashi Patent) have much lower thermal conductivities than ceramics; and, as a consequence, have not been used as circuit substrates for applications where any substantial amount of thermal conductivity is required.
The mounting of semiconductor components on ceramic substrates for improved thermal characteristics results in structures which generally are referred to as "hybrid" structures. Where ceramics such as alumina are employed, the resultant structure is relatively thick. In addition, hybrid ceramic semiconductor structures are comparatively expensive and require specialized manufacturing processes.
It is desirable to provide structures having the thermal characteristics of hybrid ceramic structures without the disadvantages of such hybrid structures. Further it is desirable to utilize flexible circuit materials employing polyimide films in place of alumina ceramic hybrid structures.